The term “filament LED” (or “LED filament”) is an industry accepted name seen, for example, in magazine articles and online discussions about a type of chip on board or glass (COB or COG) LED light source typically comprising many (sometimes hundreds) of tiny unpackaged LED chips wire-bonded as a closely spaced string on a long narrow and thin substrate. For the low power filament LEDs typical to date, this configuration has avoided the need for massive heat sinks by use of helium gas filling to enhance heat transport to the bulb by convection and conduction of the cooling gas.
FIG. 1 shows examples of prior art Osram lamps (˜4 W, 220V in A60 and ST64 bulbs) that use filament LEDs as a light source, as shown and described in the SOLERIQ® L38 Reference Design document (Osram Opto Semiconductor GmbH). Each example has four L38 filament LEDs, arranged as two series-connected pairs of parallel-connected LED filaments. This provides an axially extended light source arranged around the central lamp axis although individual filaments are typically not parallel to the axis (not axially aligned). The lamps have a fused glass seal in the neck and are ˜90% He filled to cool the LEDs by heat transfer to the bulb, which is relatively distant from the filament LEDs, as shown. In the SOLERIQ L38 Application Guide, “suitable lamp types” are listed as: A17-A19 (US)/A55-A60 (metric); ST64 (metric); C35 (metric); and G25-G40 (US). Thus the suggested bulb shapes all have vertically/longitudinally curved sides (diameter varies with longitudinal distance/height/length from the lamp base).
FIGS. 2-3 show an example schematic representation of an L38 filament LED (drawing reference number 2) with a plurality, e.g., thirty-two, of sapphire LED chips 10 connected in a linear array between anode and cathode electrical connection pins 8 (e.g., flattened metal tabs). The chips are mounted on a relatively long and narrow (e.g., 30.00×0.85 mm), rigid (e.g., aluminum) substrate 9 and the entire chip array is encapsulated in a coating of silicone 11 which has the LED phosphor mixed into it for transforming the LED output into “white” light. As shown, the light emitting chips are mounted on a “top” side of an opaque substrate, therefor most of the radiant power is emitted “outward” from the “top” side of the filament 2, i.e., generally in the “Z” direction.
Obviously for an opaque substrate, the coating 11 of silicone with phosphor is only needed on the top side, however it is known to use chip on glass (COG) technology wherein the LED chips are mounted on a light transmitting substrate such as glass and sapphire material, in which case the phosphor is also needed on the bottom side.
The SOLERIQ Application Guide recommends helium lamp fill gas to optimize LED cooling because, according to their tests: “Due to the better thermal conductivity of helium, the filament runs at a lower Tj [junction temperature]. Tj of filament LED in air is ˜1.7 times higher than in helium.”
The Application Guide lists example lamp configurations ranging from two to eight L38 filament LEDs to output lumen equivalents of 25 W to 60 W incandescent lamps.
It is an object of the inventive work disclosed hereinbelow to achieve much higher light output from a lamp, for example a lamp having the equivalent of eighteen SOLERIQ L38 filaments in a single bulb. This quantity can be expected to generate more than two times the amount of heat as an eight-filament lamp, therefor improved means of cooling the LEDs is a further objective of this inventive work.